Thermally insulating panels are typically constructed by depositing a strip of solder glass around the periphery of the glass sheets; depositing an array of support pillars onto one or other of the sheets of glass; bringing the sheets together or permitting them to move together; heating the panels to melt the solder glass around the periphery so that the two sheets settle onto the pillars; and then cooling the panel to solidify the edge seal. The panel is then evacuated by pumping out through a tube positioned either through one of the glass sheets or through the edge seal, and finally the pump-out tube is melted and sealed off.
The improvement concerns the evacuation of the panels, and in particular a method of improving the evacuation of the panels during construction, and a panel produced by the improved method.
Several conditions must be satisfied in the construction of evacuated glazing. First, it is necessary to produce an hermetic (leak-free) edge seal around the periphery of the panel. Second, in order to prevent the glass sheets from being forced into contact with each other, by the large atmospheric pressure forces acting, it is essential to include an array of support pillars within the glazing. Third, the space between the glass sheets must be evacuated to a high level, and this level of vacuum maintained over the life of the glazing. Fourth, in order to reduce radiative heat flow through the glass, a low emittance coating is applied to the inner surfaces of one or both sheets of glass.
In order to evacuate and seal evacuated glazings, it is necessary to incorporate into their structure a small pump-out tube through which evacuation occurs, The tube is necessary because, after the evacuation and bakeout of the structure, the hole through which the evacuation takes place must be sealed. It is difficult to do this directly on the surface of the glass sheets because the sealing process involves high temperatures which cause stresses in the glass sheets. The end of a small tube can be melted and sealed, however, without the necessity to heat the body of the panel which is being evacuated.
Typically a small glass tube is sealed into a hole which passes through one of the glass sheets using low melting point glass (solder glass). It has been found that using solder glass to seal an abraded glass surface results in joints which have a high probability of leakage, due to gas flow in the abraded region of the glass beneath the solder glass. It is therefore common practice to seal the tube onto the smooth unmachined inner surface of the glass sheet in order to make a leak-free joint. However, one of the problems with this, is that cracks occasionally occur in the relatively exposed solder glass seal between the pump-out tube and the glass sheet. The cracks arise because the thermal expansion of the solder glass does not exactly match that of the glass sheet. Tensile stresses therefore build up in the solder glass at the seal which may result in the formation of cracks.